Recently, multi-antenna communication such as MIMO (Multi Input Multi Output) has attracted attention as a technology that enables large volumes of data to be transmitted at high speed. It has consequently been considered possible to achieve extremely high-speed data transmission by combining OFDM (Orthogonal Frequency Division Multiplexing) and multi-antenna communication. However, data error rate characteristics degrade unless highly precise propagation path compensation and interference compensation are carried out on the receiving side.
Thus, with this kind of OFDM communication method, a received signal with good error rate characteristics can be obtained by creating pilot carriers by superimposing known signals such as pilot symbols on predetermined subcarriers on the transmitting side, as shown in FIG. 1, and compensating for propagation path distortion such as frequency offset of each subcarrier on the receiving side based on these pilot carriers.
Also, with an OFDM communication method, an OFDM signal with a propagation path estimation preamble placed on each subcarrier is transmitted by the transmitting side, and compensation of phase rotation of each subcarrier is performed on the receiving side based on this propagation path estimation preamble.
Actually, a transmitting apparatus transmits a burst unit signal such as that shown in FIG. 2, for example. As shown in FIG. 2, a burst unit signal includes guard intervals (GI), a propagation path estimation preamble, and an information signal (DATA1, . . . ). In a burst unit signal, the propagation path estimation preamble is subjected to IFFT (inverse fast Fourier transform) processing, and the information signal is subjected to predetermined modulation processing and IFFT processing.
The receiving-side apparatus detects the FFT (fast Fourier transform) processing start timing by calculating a correlation value between the IFFT-processed propagation path estimation preamble and the propagation path estimation preamble in the received burst unit signal (received signal). The receiving-side apparatus then extracts the propagation path estimation preamble and information signal from the received signal by performing FFT processing on the received signal in accordance with the detected start timing. The receiving-side apparatus also performs propagation path estimation using the extracted propagation path estimation preamble, and performs information signal demodulation using the result of propagation path estimation. By this means, the receiving-side apparatus can extract a demodulated signal.
The principle of transmission/reception by an OFDM communication apparatus using MIMO technology will now be explained using FIG. 3. FIG. 3 illustrates a case where OFDM signals are transmitted from an OFDM communication apparatus (TX) 1 that has two antennas AN1 and AN2 to an OFDM communication apparatus (RX) 2 that has two antennas AN3 and AN4. Signals transmitted from antennas AN1 and AN2 of OFDM communication apparatus 1 are here designated TX1 and TX2 respectively, and signals received by antennas AN3 and AN4 of OFDM communication apparatus 2 are designated RX1 and RX2 respectively. Received signals RX1 and RX2 can then be expressed by the following equations.RX1=ATX1 +BTX2  (1)RX2=CTX1 +DTX2  (2)
In Equation (1) and Equation (2), A denotes the propagation path characteristic between transmitting antenna AN1 and receiving antenna AN3, B denotes the propagation path characteristic between transmitting antenna AN2 and receiving antenna AN3, C denotes the propagation path characteristic between transmitting antenna AN1 and receiving antenna AN4, and D denotes the propagation path characteristic between transmitting antenna AN2 and receiving antenna AN4.
FIGS. 4(A) and (B) and FIGS. 5(A) and (B) show the frame formats of OFDM transmit signals transmitted from OFDM communication apparatus 1. FIGS. 4(A) and (B) show frame formats focusing on pilot carriers, and FIGS. 5(A) and (B) show frame formats focusing on propagation path estimation preambles. That is to say, the OFDM signal shown in FIG. 4(A) is transmitted from antenna AN1, and the OFDM signal shown in FIG. 4(B) is transmitted from antenna AN2. In FIGS. 4(A) and (B), DATA1(N,K), for example, indicates that the N'th symbol relating to data 1 is transmitted by the K'th subcarrier at the time and frequency indicated by DATA1. In FIGS. 5(A) and (B), propagation path estimation preamble (1, k) indicates that the 1st symbol of the propagation path estimation preamble is transmitted by the k'th subcarrier at the time and frequency indicated by propagation path estimation preamble (1, k).
In order to demodulate above transmit signals TX1 and TX2 from the received signals, it is necessary to estimate the four propagation path characteristics A, B, C, and D. For this purpose, OFDM communication apparatus 1 inserts propagation path estimation preambles in the transmit signals or transmits OFDM signals with specific subcarriers as pilot carriers. On receiving these OFDM signals, OFDM communication apparatus 2 obtains propagation path characteristics based on these propagation path estimation preambles or pilot carriers.
The four propagation path characteristics A through D can be estimated by OFDM communication apparatus 2 (FIG. 3) as follows. For propagation path characteristic A, a propagation path estimation preamble transmitted from antenna AN1 is received at antenna AN3, and propagation path characteristic A is found by a signal processing section corresponding to antenna AN3. For characteristic B, a propagation path estimation preamble transmitted from antenna AN2 is received at antenna AN3, and characteristic B is found by the signal processing section corresponding to antenna AN3. For characteristic C, a propagation path estimation preamble transmitted from antenna AN1 is received at antenna AN4, and propagation path characteristic C is found by a signal processing section corresponding to antenna AN4. For characteristic D, a propagation path estimation preamble transmitted from antenna AN2 is received at antenna AN4, and characteristic D is found by the signal processing section corresponding to antenna AN4.
OFDM communication apparatus 2 can perform demodulation of signals TX1 and TX2 transmitted from antennas AN1 and AN2 by performing the processing shown in the following equations, using the four estimated propagation path characteristics A through D.
                                          DRX1            ⁡                          (                              AD                -                BC                            )                                -                      BRX2            /                          (                              AD                -                BC                            )                                      =                                                            D                ⁡                                  (                                      ATX1                    +                    BTX2                                    )                                            /                              (                                  AD                  -                  BC                                )                                      -                                          B                ⁡                                  (                                      DTX1                    +                    DTX2                                    )                                            /                              (                                  AD                  -                  BC                                )                                              =                                                    (                                  ADTX1                  +                  BDTX2                  -                  BCTX1                  -                  BDTX2                                )                            /                              (                                  AD                  -                  BC                                )                                      =            TX1                                              (        3        )                                                                    -              CRX1                        /                          (                              AD                -                BC                            )                                -                      ARX2            /                          (                              AD                -                BC                            )                                      =                                                            -                                  C                  ⁡                                      (                                          ATX1                      +                      BTX2                                        )                                                              /                              (                                  AD                  -                  BC                                )                                      +                                          A                ⁡                                  (                                      CTX1                    +                    DTX2                                    )                                            /                              (                                  AD                  -                  BC                                )                                              =                                                    (                                                      -                    ACTX1                                    -                  BCTX2                  +                  ACTX1                  -                  ADTX2                                )                            /                              (                                  AD                  -                  BC                                )                                      =            TX2                                              (        4        )            
Propagation path estimation preambles are actually transmitted as follows. A propagation path estimation preamble is not transmitted from antenna AN2 during the time when a propagation path estimation preamble is being transmitted from antenna AN1. Similarly, a propagation path estimation preamble is not transmitted from antenna AN1 during the time when a propagation path estimation preamble is being transmitted from antenna AN2.
In general, a pilot carrier is used to compensate for residual phase error due to frequency offset detection error, etc. That is to say, during reception, residual phase error is detected using a known signal (pilot signal) multiplexed in a pilot carrier, and compensated for. Actually, specific subcarriers are transmitted as pilot carriers, as shown in FIGS. 4(A) and (B). In the example shown in FIGS. 4(A) and (B), of 2k+1 subcarriers, four antenna AN1 subcarriers are transmitted as pilot carriers.
FIG. 6 shows the configuration of the transmitting system of OFDM communication apparatus 1. In transmitting system 10, a transmit signal is first coded by a coding section 11. The coded signal is subjected to preamble insertion by a preamble insertion section 12, and is then subjected to insertion of a known signal (pilot signal) by a pilot carrier insertion section 13 at positions at which specific subcarriers are pilot carriers.
After undergoing modulation processing by a modulation section 14, the signal is divided into two by being subjected to serial/parallel conversion by a serial/parallel conversion section (S/P) 15. The two divided signals undergo inverse fast Fourier transform processing by inverse fast Fourier transform sections (IFFTs) 16 and 17 respectively, thereby being orthogonal frequency division multiplexed by IFFTs 16 and 17, and OFDM signals are obtained. IFFT 16 output signal 1 is superimposed on a carrier of a predetermined frequency by a radio transmitting section (not shown), and then transmitted from antenna AN1 (FIG. 3). Similarly, IFFT 17 output signal 2 is superimposed on a carrier of a predetermined frequency by a radio transmitting section (not shown), and then transmitted from antenna AN2 (FIG. 3).
FIG. 7 shows the configuration of the receiving system of OFDM communication apparatus 2 (FIG. 3). In receiving system 20, a received signal received by antenna AN3 is input via a radio receiving section (not shown) to a fast Fourier transform section (FFT) 21 as input signal 1, and a received signal received by antenna AN4 is input via a radio receiving section (not shown) to a fast Fourier transform section (FFT) 22 as input signal 2.
FFT 21 obtains a received signal for each subcarrier by executing fast Fourier transform processing on input signal 1. The received signals of each subcarrier obtained by FFT 21 are sent to a propagation path estimation section 25, and propagation path compensation and interference compensation sections 24 and 26. Input signal 2 is converted to received signals for each subcarrier by FFT 22, and these signals are sent to propagation path estimation section 25, and propagation path compensation and interference compensation sections 26 and 24.
Propagation path estimation section 23 estimates propagation path characteristics A and B described above with regard to FIG. 3 based on the preambles inserted in the received signals. Similarly, propagation path estimation section 25 estimates propagation path characteristics C and D based on the preambles inserted in the received signals.
A coefficient calculation section 27 uses propagation path characteristics A through D obtained by propagation path estimation sections 23 and 25 to find coefficients A/(AD−BC), B/(AD−BC), C/(AD−BC), and D/(AD−BC). Coefficient calculation section 27 is configured as shown in FIG. 8. The four propagation path characteristics A to D obtained by propagation path estimation sections 23 and 25 are stored in memories 41 to 44 respectively. AD is obtained by a multiplication section 46, and BC is obtained by a multiplication section 45. AD−BC is obtained by a subtraction section 47. A/(AD−BC), B/(AD−BC), C/(AD−BC), and D/(AD−BC) are obtained by division sections 48, 49, 50, and 51, respectively.
We will now return to FIG. 7 to continue the explanation. Propagation path compensation and interference compensation section 24 forms a received signal TX1 that has undergone propagation path compensation and interference compensation by performing the computation shown in Equation (3) on the received signals using the coefficients found by coefficient calculation section 27. Similarly, propagation path compensation and interference compensation section 26 forms a received signal TX2 that has undergone propagation path compensation and interference compensation by performing the computation shown in Equation (4) on the received signals using the coefficients found by coefficient calculation section 27.
Received signal TX1 that has undergone propagation path compensation and interference compensation is sent to a residual phase error detection section 28 and phase compensation section 29, and received signal TX2 that has undergone propagation path compensation and interference compensation is similarly sent to residual phase error detection section 28 and phase compensation section 30. Residual phase error detection section 28 detects residual phase error in the two received signals TX1 and TX2 using a known signal transmitted by pilot carriers, and sends this to phase compensation sections 29 and 30.
Phase compensation sections 29 and 30 perform phase compensation processing by rotating the phase by the residual phase error amount for received signals TX1 and TX2 respectively. The two phase-compensated received signals are converted to a serial signal by a parallel/serial conversion section (P/S) 31, and a received signal corresponding to the transmit signal is obtained by decoding this serial signal in a decoding section 32.
However, with a conventional OFDM communication apparatus, as can be seen from FIGS. 4(A) and (B), data transmitted from one antenna is superimposed as interference on known signals (pilot carriers) transmitted from the other antenna. Therefore, the interference component superimposed on known signals must be eliminated in order to detect residual phase error.
However, when inter-code interference, timing error, and frequency offset detection error are present due to multipath propagation, interference elimination characteristics degrade. As a result, an interference component remains in known signals, causing a problem of major degradation of error rate characteristics.
Moreover, with a conventional OFDM communication apparatus, as shown in FIGS. 5(A) and (B), the time at which a propagation path estimation preamble is transmitted differs for transmitting antenna AN1 and transmitting antenna AN2.
Consequently, if there is residual phase error in received signals RX1 and RX2 obtained by the two receiving antennas AN3 and AN4, there will be residual phase error in the propagation path estimation results estimated with the propagation path estimation preambles. When residual phase error is present, that residual phase error becomes propagation path estimation error, resulting in major degradation of error rate characteristics on the receiving side. Thus, a defect of this kind of conventional OFDM communication apparatus is that error rate characteristics degrade significantly when residual phase error is present.